The utilization of molecular compounds in nanoelectronics as a working active material is largely founded upon to use specific properties of thin and over thin films created on their basis. From one point of view, new technology, searching and the research of new physical effects in thin molecular films represents large interest. One of them are the effects of switching and memory. There are in view of the effects of the sharp change of electrical conductivity film in the direction of the perpendicular plane of one, under action of an electrical field. The electrical switching and memory phenomena are observed in molecular thin films sandwiched between two metal electrodes which are called a memory cell. The Memory Cell (MC) can be used for creating digital and analogue memory chips (DRAM, SRAM, EPROM and so on).
The observation of electrical switching and memory phenomenon is known to exist in a wide variety of inorganic and organic materials. In spite of the fact that the first publications on these subjects occurred long ago, first researches relate to the sixties, so far the unequivocal interpretation of similar phenomena is not present. This partly connected with the unreproducibility of the results of different authors presented for the same compound, but also frequently the bad effects are reproduced by the same authors. There is a good probably that different explanations of the switching may take place for the various molecular films. In these materials the switching and memory phenomena are given rise to by the field assisted by structural changes such as phase transitions, crystallization, metal filament formation which are assisted by highly localized Joule heating. Electrical behavior of these materials is not stable, reproducible, independent from polarity and usually requires the high voltage of the switching.
From the point of view of construction of simple and effective memory chips, it would be very interesting to find the switching mechanism and memory occurring on the level of molecule or molecular ensemble and having macroscopic exhibition. There is a small number of such phenomena. One of them is the effect of electronic instability (Peierls' effect) in one-dimensional molecular systems. It should be noted that in most cases the molecular films which were studied were one-dimensional molecular system. The mechanism of the switching and memory which is suggested by us is based on the intramolecular quantum phenomena which are a result of atomic and molecular motions (Krieger 1993, Krieger 1996).
In this paper the electrophysical characteristic of the memory cell and possibility of using this property of MC for the creation of memory chips are investigated. (Krieger, Yudanov et al, 1993).
The appearance and internal organisation of the memory cells are shown in Fig. 1 a,b. A layer of copper conductors with a chromium sublayer are evaporated through a mask onto a cleaned glass slide. Over the copper conductors, the molecular film was produced. As a molecular compound, the following typical molecular one-dimensional systems were used: polyconjugated compounds (phenylacetylene), Cu-phtalocyanine and charge transfer complex (TCNQ). The method of preparation of thin films is described earlier. Aluminium conductors are then evaporated through a mask onto the molecular film orthogonally to the copper ones, producting the MC active areas of 100x100 micron2 (Fig. 1.a). In order to get the active areas of the memory cell of 2x2 micron2 or 0.2x0.2 micron2 and under, we used a different type of MC (Fig. 1.b). Electrode thickness was approximately 0.5 micron, while the molecular films thickness was in the range from 0.1-0.5 micron.
To measure electrophysical MC characteristics we used the circuit shown in Fig. 3.
All the memory cells were characterised by current-voltage (I-U) measurements. Switching (I-U) characteristics were displayed on an X-Y recorder or oscilloscope. The main attention was paid to the reproducibility of the switching effect and the switching times and the storage time. In order to strive for higher reliability and replication, it is necessary to use a load resistor (RL). A series resistor was used to limit current in the transition to the ON state. In all measurements, the common electrode is aluminium. The positive voltage with respect to the ground, applied to the copper electrode, will be refered to as a write voltage (in pulse experiment, as a write pulse (PW)) and the negative as erase voltage (as erase pulse (PER) , accordingly). In all reading experiments to the memory cells, voltage amplitude, which does not change its state, did not exceed 50 mV. In the process of measurements it became clear that threshold voltage depends on the previous state of the cell. In order to remove the previous influences on the memory cell parameters, it was brought to the equilibrium state, which is characterized by high impedance. The main experiments are conducted with MC forming an memory cell array (8x8) as shown on a Fig. 2. and using phenylacetylene as work compound.
Electric switching phenomena in our molecular thin films, as in other works, are characterized by the existence of two stable states, a high impedance state ("OFF" state) and a low impedance state ("ON" state). The impedance of this "OFF" state was usually more than 20 megohms. Switching from the "OFF" state to the 'ON' state occurs when an increasing electrical field exceeds a threshold value. The impedance of this "ON" state was less than 10 ohms. A transition from "ON" state to the "OFF" state takes place when an electrical field of different polarity is applied.
Fig. 4 shows a typical DC (I-U) curve. The measurement results have demonstrated that the cell possesses an S - shaped (I-V) characteristic with memory. For some cells, an extraordinary dc (I-U) dependence is observed.
The experimental timing diagram of the write and erase of the pulse mode is shown in Fig. 5. Before each recording the cell is brought to the equilibrium state. The operating region of the PW and PER for the memory cell is limited to about 1 µs (Fig. 6). For some cells, this region extends by about 10 ns. As a rule the memory cells were still switching after more than 108 cycles.
The following types of memory are observed in the memory cell and it depends on the type of molecular system and the switching condition:
the "ON" state is conserved as long as an external field is applied and the film returns to its initial high impedance (the "OFF" state) after the applied electrical field is removed.
the "ON" state is conserved from a few seconds up to several month without the external electrical field and can be removed in the "OFF" state by the application of a short field pulse of a different polarity.
the "ON" state is conserved as long as necessary and requires the special electrical pulse for switching to the "OFF" state.
Any arbitrary impedance of the memory cell could be obtained in the interval between 20 megohms and 10 ohms depending on the switching condition. In order to take the state of the memory cell with different intermediate impedance, it is necessary to use a different load resistor. As a rule, the impedance of the cell after switching to the "ON" state is in proportion to the load resistor. Fig. 7 shows the memory cell switching as a function of the load resistor.
In addition, the state of the cell with the various impedance can be taken by pulse experiments, too. It was found the time storage of the memory cells are inversely proportional to the RMC. Moreover, we can identify two mode of the MC operation: the stable mode and metastable mode. The stable mode of the MC operation shows the high PW and PER value (3-10V), low impedance of the "ON" state (less than 10 ohm), long time switching (1 ms and more) and long time storage (more than two months). On the contrary, the metastable mode of the MC function is characterized by the low PW and PER value (0.1-0.5V), high impedance of the "ON" state (wide region, about 1kiloohm - 1megohm), short time switching (less then 1 µs and more) and short time storage (about ten second or some hours). (Fig. 8). The memory cells store the recorded state from some second to several months. The memory cells easing reproduce electrophysical property after storage for three years.
In view of these results, we believe that on the basis of our molecular memory cells it is possible to carry out the memory chips for computer systems. The circuit of the molecular memory chip is based on well developed chips of SRAM with capacitors. In this circuit the capacitor is excluded and replaced on our molecular memory cell by one. As a result, the area occupied by memory elements is reduced. The uses of this circuit simplifies the manufacturing of chips and provides a greater output of ready production. In accordance with the molecular compound and the regime of write and erase, this circuit can work as either SRAM or DRAM. The information is defined on the value of memory cell resistance. The resistance principle of the information recording makes it possible to take a multi bit writing and reading of the information and design Molecular Analogue Memory Cells (MAMC).
In our opinion the given method allows an increase in the information density by up to 2-4 times. Potentially, by further updating this method, an information density of 8 bits and more per one memory cell can be achieved. This increase in the information density will be made only by changing the write and erase regime. Consequent development of a molecular transistor allows the creation of arrays of memory cells on polymeric films, that will enable the designing of multilayer volumetric memory devices. Moreover, development of the given principle of information storage allows the creation of analogue plastic synapses of a neuron net for a future generation of computers. The absence of such an element constrains the existence of neuron computers. The analogue memory cells correspond in the best way to the learning and functioning conditions of neuron synapses. Furthermore, MAMC has no sensitivity to radiation, i.e. the information stored therein cannot leak away like information stored on a floating gate device.
MAMC can be used both for creating digital memory chips with one bit or multibit modes for writing information (DRAM, SRAM, EPROM and so on) and for designing analogue memory chips and analogue plastic synapses for neuron networks. The absence of such element constrains the occurrence of real neuron computers. Using these memory cells will lead to a new generation of memory technology for the next millennium.
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Krieger Ju.H. (1996), Electronic instability of one-dimensional
molecular system as physical principle for design of electronic
devices. Extended Abstracts (The Third European Conference on
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Krieger Ju.H., Yudanov N.F., Igumenov I.K., Vaschenko
S.B. (1993). Study of test structures of molecular memory element.
J. Struct. Chemistry 34 pp. 966-970.
Dr. Juri H. Krieger
Institute of Inorganic Chemistry
Lavrentiev ave. 3, 630090, Novosibirsk, Russia